The present invention generally relates to apparatus and methods for forming nanolaminate thermal barrier coatings for turbine engine blades and vanes and more specifically to apparatus and methods for forming thermal barrier coatings composed of a series of non-homogeneous nanometer—to micron-sized layers for turbine blades or vanes.
Increased gas turbine engine operating temperatures often result from efforts to improve on overall engine efficiency as well as to reduce emission of contaminants released to atmosphere. Increased operating temperatures, however, result in concerns over the ability of various engine components, such as turbine blades and vanes, to remain durable and maintain their mechanical strength. These concerns have been addressed in the following two principal ways: first, the formulation of superalloys, such as nickel and cobalt based, having high temperature capabilities; and, second, the application of protective thermal barrier coatings (TBC's) which insulate the components thus minimizing their service temperatures.
Referring to the application of protective coatings, it is known that the characteristics of TBC's must include the capability to strongly adhere and remain adhered to the component to which it is applied, and low thermal conductivity. Typically, TBC's applied to superalloy substrates have included a bond coat and a ceramic top layer, the latter being applied either by the process of plasma spraying or by the process of electron beam physical vapor deposition (EB-PVD). Use of the EB-PVD process results in the outer ceramic layer having a columnar grained microstructure. Gaps between the individual columns allow the columnar grains to expand and contract without developing stresses that could cause spalling. Prior art has disclosed thermal barrier coatings for superalloy substrates that contain a MCrAlY layer, an alumina layer, and an outer columnar grained ceramic layer. Also TBC's for superalloy substrates have included those that contain an aluminide layer, an alumina layer, and an outer columnar grained ceramic layer with the ceramic layer commonly being zirconia stabilized with yttria.
U.S. Pat. No. 4,676,994 to Demaray discloses that a layer of dense stabilized zirconia can be deposited onto the bond coating when the gas pressure within the chamber is less than 0.0001 torr. Subsequent injection of oxygen at a pressure of 0.0001 to 0.01 torr into a stabilized zirconia vapor cloud increases the oxygen content of the ceramic layer and initiates growth of substantially stoichiometric columnar ceramic grains with intercolumnar porosity for strain tolerance. Tubes and nozzles are used to direct the oxygen gas to impinge upon the substrate.
U.S. Pat. No. 5,534,314 to Wadley et al. discloses the use of a carrier gas to entrain the evaporant in the carrier gas stream and coating the part with the carrier gas stream containing the entrained evaporant. Depending upon the requirements of the coating, the carrier gas may be oxygen, nitrogen, helium or another inert gas such as, methane or acetylene. The carrier gas is used to increase the deposition rate onto the substrate and, because gas is used with associated pressure (0.001 torr to 1 atmosphere), the coating is more uniform on complex shaped parts and less line of sight limited. The process disclosed by Wadley does not deal with nanolaminates.
U.S. Pat. No. 6,054,184 to Bruce et al. discloses a method and apparatus for forming a multilayer thermal barrier coating such that the coating is composed of substantially homogeneous layers of different materials. The process requires two ceramic vapor sources separated by a baffle to make multi-layer TBC's composed of homogeneous successive layers of stabilized zirconia+alumina. The use of a baffle significantly reduces the efficiency of deposition, as much of the vapor cloud is wastefully deposited on the baffle.
U.S. patent application Ser. No. 09/535,394, filed Mar. 24, 2000, which Applicant hereby incorporates herein by reference, filed on behalf of the assignor herein, discloses a thermal barrier coating which includes a columnar grained ceramic layer applied to an aluminide or MCrAlY bond coat by EB-PVD. The ceramic layer is comprised of a plurality of layers of zirconia stabilized with 20 percent yttria and the interfaces between the layers are decorated with particles selected from a group consisting of Ta2O5 and alumina. A baffle is not required but is optional in the disclosed process which is aimed at producing a lower conductivity thermal barrier coating.
Although prior art has resulted in various improved methods and apparatus for thermal barrier coatings, none results in columnar grained thermal barrier coatings composed of a multitude of non-homogeneous, nanometer—to micron size, successive layers separated by non-homogeneous interfaces stabilized by nanometer-sized second phase particles.
As can be seen, there is a need for an improved apparatus and method for forming non-homogeneous nanometer—to micron-sized multi-layer thermal barrier coatings for superalloy substrates such as turbine blades or vanes.